How do I make a hollow sphere?
It looks as if you already used a cube with subdivision surface modifier to get your sphere. While not a bad idea, i'd recommend to use at least subdivision level 4 for a cleaner sphere, otherwise it's not really as spherical as you'd want.
Thanks to Trey Harper for pointing out the cast modifier, i barely use, which in this case though really improves the result.
That said, going from your (cube)sphere:

Apply the subdivision surface modifier on your cube(remember lvl 4, for a more spherical sphere) and a cast modifier with the factor of 1:

Marked red, important before going on is to apply these modifiers in order!
Go in front view Num 1, activate edit mode Tab
and x-ray mode Alt+Z, select half of the sphere like this:

when selecting, leave the center deselected to keep a half
With the selection, press X and choose Vertices
In the Object Data Properties we need to activate the Normals&gt;Auto Smooth:

Auto Smooth is needed for the next step to look clean at the cut
Go back in object mode Tab and add a solidify modifier.

Adjust the modifier to your needs and you have one shell done.

You can now copy Shift+D the created shell and adjust it's size to gain the other layers of the planet.
Here a little visual demonstration:

The thickness of each layer can easily be adjusted in the modifier
As the core looks like a full sphere, you should have all needed means to your cause now.
Happy Blending